1. Field of the Invention
The present invention relates to an in-cylinder direct-injection spark-ignition internal combustion engine, and specifically to techniques for an intake port structure capable of providing the enhanced intake-air charging efficiency and ensuring the excellent air/fuel mixture formation, and suitable for a cylinder direct-injection spark-ignition engine, capable of operating in at least two modes, namely a homogeneous combustion mode and a stratified combustion mode.
2. Description of the Prior Art
In recent years, there have been proposed and developed various in-cylinder direct-injection spark-ignition engines in which fuel is injected directly into an engine cylinder. Generally, on such direct-injection spark-ignition engines a combustion mode is switchable between a homogeneous combustion mode (or an early injection combustion mode) where fuel-injection early in the intake stroke produces a homogeneous air-fuel mixture, and a stratified combustion mode (or a late injection combustion mode) where late fuel-injection delays the event until near the end of the compression stroke to produce a stratified air-fuel mixture. The previously-noted homogeneous combustion mode is suitable for an engine operating condition such as medium or high engine speed and load, whereas the previously-noted stratified combustion mode is used generally at an engine operating condition such as low engine speed and load. The purpose of the stratified combustion is to deliver a readily ignitable mixture (richer mixture of a combustible air/fuel mixture ratio at which the mixture is ignitable by means of a spark plug provided in the combustion chamber) in the vicinity of the spark plug while forming surrounding air layer (leaner or ultra-leaner mixture often including part of the exhaust gas back through the engine or having the difficulty of direct-ignition by the spark plug) that contains little fuel, and to stable lean combustion under the condition of the low engine speed and load, and to improve fuel economy. In contrast to a conventional engine, the throttle valve is opened at the low engine load to increase the intake-air quantity, thus reducing a pumping loss. On such cylinder direct-injection spark-ignition engine (simply a DI engine) has been disclosed in Japanese Patent Provisional Publication Nos. 62-191622 and 2-169834. On the other hand, Japanese Patent Provisional Publication No. 7-119472 teaches the provision of a so-called swirl control valve to modulate in-cylinder gas motion and to create turbulent flow such as swirl flow or tumble flow, thus facilitating the mixing of air with fuel spray and improving combustion.
During the homogeneous combustion mode (or the early injection combustion mode) on the intake stroke, it could be expectable to provide the effect of cooling the intake air owing to fuel vaporization (or gasification), thus ensuring an enhanced intake-air charging efficiency (simply an induction efficiency) arising from reduction in the volumetric capacity of intake air charged. However, in conventional engines, the effects (the enhanced intake-air cooling effect and the enhanced induction efficiency) as previously discussed, were inadequate and unsatisfactory. It would be possible to highly enhance the intake-air cooling effect by promoting the fuel vaporization by virtue of increase in the flow-velocity of intake air flowing through the intake port. In addition to the above, the increase of the flow-velocity of intake air can contribute to strengthen turbulent action (for example tumble flow) in the engine cylinder, thereby assuring good mixing of fuel sprayed out with air and thus promoting homogenization of the air-fuel mixture within the combustion chamber. Also, it is possible to properly carry the fuel spray to the vicinity of the spark plug during the stratified combustion mode by swirl motion more greatly strengthened by virtue of both the increased intake-air velocity and the use of the swirl control valve. This may largely improve combustion characteristics (for example, the fuel consumption, the lean misfire limit, and the combustion stability) during the stratified combustion mode. For the reasons set forth above, it would be desirable to effectively increase an intake-air velocity by improving an intake port structure of a direct-injection spark-ignition engine.